A class-D amplifier, as known to the prior art, uses a square wave oscillator to generate an ultrasonic frequency carrier signal. The ultrasonic carrier signal in turn is integrated to form a triangular wave carrier signal. An audio signal is summed with this triangle wave carrier signal to generate a composite signal. The composite signal is applied to a comparator. The comparator generates a pulse width modulated (PWM) output signal in which the width of the pulse encodes the audio signal information.
In more detail, an example of such a class-D pulse width modulated amplifier known to the prior art is shown as a block diagram in FIG. 1. In this embodiment, a ultrasonic output square wave 10 of a square wave oscillator 12 is integrated by an integrator 14 to form a triangular wave carrier 16. The triangular wave carrier 16 is summed by a summer 22 with an audio signal 24 from an audio input 26 and the resulting composite signal 28 is applied to one input 30 of a comparator 32. The other input 34 of the comparator 32 is connected to a reference voltage 36. The output terminal 40 of comparator 32 is connected to the input terminal 42 of an inverter 44. The output terminal 46 of inverter 44 is connected both to one terminal 48 of a load 50, such as an audio transducer, and the input terminal 52 of a second inverter 54 whose output terminal 56 is connected to the second input terminal 58 of load 50.
As can be seen, the result of the summation of the audio signal and the triangular carrier signal is a modulated carrier which moves above and below the reference voltage level of the comparator. As a result, the output of the comparator will transition high as the composite signal exceeds the reference voltage level and will transition low when the composite signal goes below the reference voltage level.
Thus, the output of the comparator will be a series of pulses, the width each being determined by the length of time the composite signal spends above the reference voltage level. The series of pulses thus reflects the audio signal which formed the composite signal and is therefore a pulse width modulated analog of the audio signal.
Although the transducer load is exposed to both the audio signal and the ultrasonic carrier signal in the form of the modulated width pulse train, only the low frequency audio signal is converted to sound since the audio transducer load is primarily inductive. That is, since the impedance of the load increases with frequency, the high frequency portion of the signal is not converted to sound but is reflected back to the battery.